Recent work has demonstrated that viruses have a complex and antagonistic relationship with host cells. Viruses have evolved proteins to exploit host proteins and cellular pathways to guarantee successful infection. In turn, host cells have evolved restriction factors to directly inhibit viral replication. As a result of this ancient and on-going conflict, the host and viral genes at the center of the conflict are evolving under positive selection. By studying the evolutionary history of host genes, we can identify new host antiviral genes and uncover the nature of their interaction with viruses. I have used evolutionary approaches to demonstrate that the gene, Zinc-finger Antiviral Protein, or ZAP, is evolving under positive selection in primates, suggesting it has a role in primate host defense. Further, my work implicates the PARP domain of ZAP as being at the interface of the conflict between ZAP and the virus. My proposal focuses on using functional assays to determine if human ZAP has antiviral activity and more specifically to address the role of the PARP domain in viral restriction. I will test primate ZAP isoforms with and without the PARP domain in cells challenged by MLV or HIV infection. I will also generate a version of human ZAP with a catalytically dead PARP domain to determine whether poly(ADP)ribosylation activity is necessary for restrictive capabilities. These studies will help to elucidate the mechanism by which ZAP restricts viruses and provide a model for poly(ADP)ribosylation in host defense. There are 16 other PARP containing genes and given the role of ZAP in host defense, I have performed a cursory survey of other PARP genes to identify other antiviral candidates. Indeed, 5 of these genes show signatures of positive selection and the other goal of my research plan is to perform a thorough evolutionary analysis of these genes in order to better understand the selective forces driving their evolution. RELEVANCE TO PUBLIC HEALTH Using the power of evolutionary methods and functional assays to explore the host-virus conflict will reveal how viruses successfully infect cells and how host cells are able to prevent viral infection. The results of this work will shed light on efforts to design antiviral therapeutics in the treatment of AIDS and other viral diseases. [unreadable] [unreadable] [unreadable]